1. Field of the Invention
The present invention relates to a surveillance method of an optical amplifier-repeater transmission system that is designed to survey troubles in optical repeaters and optical fiber transmission lines in a long-distance relay system using optical amplifiers such as a submarine optical fiber cable system.
2. Description of the Related Art
An optical communication system using optical fiber cables is applied to a long-distance transmission system such as a submarine optical fiber cable system; and if trouble occurs in such a submarine optical fiber cable system, a quick recovery is strongly required because of the potential huge influence to the information society.
Generally, the submarine optical fiber cable system is composed of optical repeaters and optical fiber cables, and the cable can possibly be damaged by a human activity such as fishing or anchoring and the optical repeater can fall in trouble due to a failure of the electric circuit. In order to eliminate such a trouble in its earliest stages, it is most important to locate the optical cable or optical repeater fallen in trouble.
Conventionally in the optical communication system, the regenerative relay system is used which a light signal transmitted through an optical fiber cable is transformed into an electric signal in an optical repeater to perform waveform shaping and reproducing, and the electric signal is again transformed into a light signal to be sent out. Recently, however, an optical amplifier-repeater transmission system has been developed in which a light signal transmitted through an optical fiber cable is directly amplified to be sent out by an optical amplifier.
As a surveillance method of the transmission line that exploits a property of the optical amplifier-repeater transmission system to amplify a light signal optically directly without regeneratively repeating, an optical turn method is disclosed (for example, in Japanese Patent Official Publication, JP-A-5-344067 and JP-A-6-204949).
This surveillance method can detect a repeater section that is in trouble by always circulating a turned signal for surveillance from the optical repeater via the transmission line. This optical turn method will be described with reference to a block diagram showing a construction of the optical amplifier-repeater transmission system shown in FIG. 6(A). The optical turn method is designed to detect troubles in the repeaters or in optical fibers between the repeaters, whereby a light signal transmitting on one of the transmission lines and having surveillance signal superimposed is turned with a specific attenuation to the other transmission line.
In FIG. 6(A), it is assumed that the direction facing to a receiving terminal station 124a from a transmitting terminal station 123a is the ascent and the direction facing a receiving terminal station 124b from a transmitting terminal station 123b is the descent. 112a.about.112z are ascent optical fiber transmission lines, 113a.about.113z are descent optical fiber transmission lines, 116a.about.116n are optical repeaters provided at each specific distance on the ascent optical fiber transmission lines 112a.about.112z and the descent optical fiber transmission lines 113a.about.113z, 117n and 118n are optical amplifiers provided in the optical repeater 116n, 119n and 120n are optical couplers provided in the optical repeater 116n, 121n and 122n are attenuators provided in the optical repeater 116n, An is an optical turn circuit composed of the optical coupler 119n, attenuator 121n, optical coupler 120n, and attenuator 122n connected in a loop, 123a and 123b are transmitting terminal stations, and 124a and 124b are receiving terminal stations.
Furthermore, the optical repeaters 116a.about.116n are all identical in circuit, and only the optical repeater 116n is illustrated in detail.
In the optical amplifier-repeater transmission system thus constructed, each of the transmitting terminal stations 123a, 123b sends out a main line light signal, on which a surveillance signal is superimposed, as a transmission signal to each of the ascent and descent optical fiber transmission lines 112a, 113z. Each of the main line light signals sent out to the ascent and descent optical fiber transmission lines 112a, 113z is repeatedly optically amplified in the optical repeaters 116a.about.116n, and transmits through the ascent optical fiber transmission line 112a.about.112z or the descent optical fiber transmission line 113z.about.113a to be received by each of the receiving terminal stations 124a, 124b.
The main line light signals are attenuated to a specific level transmitting through the optical turn circuits Aa.about.An provided in the optical repeaters 116a.about.116n, and are turned to the facing optical fiber transmission lines. Each of the turned signals transmits through the ascent optical fiber transmission line 112a.about.112z, or the descent optical fiber transmission line 113z.about.113a to the corresponding receiving terminal station 124a or 124b on the transmission lines, respectively.
Next, how the light signal is turned will be described in an example of the optical repeater 116n. The main line light signal transmitted through the ascent optical fiber transmission line 112n is optically amplified by the optical amplifier 117n to enter the optical coupler 119n. A part of the main line light signal is split in the optical coupler 119n, and attenuated to a specific level by the attenuator 122n to enter the descent optical coupler 120n. The light signal thus attenuated is superimposed as a turned signal on the descent main line light signal transmitting through the descent optical fiber transmission line 113n. Thus, the ascent main line light signal is turned to the descent optical fiber transmission line 113n.
This function is the same in the descent main line light signal. Namely, the main line light signal transmitted through the descent optical fiber transmission line 113(n+1) is optically amplified by the optical amplifier 118n to enter the optical coupler 120n. A part of the main line light signal is split in the optical coupler 120n, and attenuated to a specific level by the attenuator 121n to enter the ascent optical coupler 119n. The light signal thus attenuated is superimposed as a turned signal on the ascent main line light signal transmitting through the ascent optical fiber transmission line 112(n+1). Thus, the descent main line light signal is turned to the ascent optical fiber transmission line 112(n+1).
Assuming that the coupling attenuation of the optical coupler 119n and 120n is designed to be, for example, 10 dB, and the attenuation of the attenuator 121n and 122n is designed to be, for example, 25 dB, the light signal transmitted through the ascent optical fiber transmission line 112n is attenuated by 45 dB to be turned to the descent optical fiber transmission line 113n as a turned signal. In the same manner, the light signal transmitted through the descent optical fiber transmission line 113n is attenuated by 45 dB to be turned to the ascent optical fiber transmission line 112n as a turned signal.
In this case, extracting the surveillance signal out of the turned signal and measuring the signal intensity enables the detection of the repeater section in which the optical repeaters or optical fiber cables are in trouble.
As a means to extract the surveillance signal out of the turned signal at the receiver stations 124a, 124b, a method is applied which finds an autocorrelation coefficient between the surveillance signal superimposed on the transmission signal at the transmitting terminal stations 123a, 123b and the received turned signal.
Thus, extracting the surveillance signal out of the turned signal at the receiving terminal stations 124a, 124b gives high level surveillance signals at each delay time corresponding to each location of the optical repeaters. The peak levels of the surveillance signals indicate turned signals from the optical repeaters. Continuously measuring (surveying) the peak levels corresponding to the locations of the repeaters and investigating level variations by comparing these peak levels with the levels measured immediately after the optical fiber cable was installed leads to detecting troubles in the optical repeaters or optical fiber transmission lines.
The surveillance method of optical amplifier-repeater transmission system shown in FIG. 6(A), however, has the following problems. FIG. 6(B) illustrates a state wherein turned signals La.about.Ln are superimposed on the ascent or descent main line light signal LCH in this surveillance method. Although, in this figure, the main line light signal LCH and the turned signals La.about.Ln are shown in different wavelengths, this is only for easy understanding, practically they have an identical wavelength. Because the main line light signal and turned signals are regulated to have an identical wavelength, the turned signal must be reduced to a level that does not give any influence to the main line light signal (attenuation requirement).
Furthermore, the surveillance signal superimposed on the main line light signal is regulated to have a low modulation rate so as not to influence the main line light signal, and therefore, received surveillance signals at the receiving terminal stations 124a, 124b become very low levels. Therefore, it takes long time to receive a signal of good S/N ratio from among weak turned surveillance signals.
To solve this problem, providing an exclusive surveillance channel can raise the modulation rate of the surveillance signal and can receive the surveillance signal of a good S/N ratio in a shorter time. However, in the wavelength-multiplexed optical transmission system aiming at increasing the transmission capacity by multiplexing multiple optical transmission signals, to avoid interference between a light signal of the exclusive surveillance channel and a light transmission signal, it becomes necessary to secure a wavelength band for the exclusive surveillance light signal in the limited light transmission band in the same manner as the light transmission signal. Consequently, a band has to be allocated for the exclusive surveillance signal, whereas the band should be used primarily for the transmission signal. Therefore, the transmission capacity will be limited, which lowers the economic efficiency.
This is stated also in "Supervisory signal transmission method for optical amplifier repeater system", IEEE GLOBECOM '90, 903.2, 1990.
Still, allocating the surveillance signal outside the band can be considered, however, it is generally known that the wavelength gain characteristics of the optical amplifier varies depending on the magnitude of input light power and the excited light intensity of an optical fiber amplifier. Generally, in a repeater system with optical amplifiers of identical characteristics connected in multiple stages, the wavelength gain characteristics are accumulated; and the multi-stage connection confines the possible repeater band, and a large gain difference occurs between a gain at a wavelength giving the peak gain and a gain at a wavelength at band edges. In view of these points, assuming the measurement of the band edge or outside of the band to be the gain characteristics within the band of the optical amplifier is imperfect in surveying the system.